Top C++ Exception Handling Interview Questions and Answers

1. What is exception handling in C++?

   • Answer: Exception handling is a mechanism in C++ that allows you to gracefully handle runtime errors (exceptions) that might occur during program execution. It separates error-handling code from the main program logic, improving code readability and maintainability. This is achieved using `try`, `catch`, and `throw` keywords.

2. What are the keywords used in C++ exception handling?

   • Answer: `try`, `catch`, and `throw` are the primary keywords. `try` defines a block of code where exceptions might occur. `throw` throws an exception object when an error is detected. `catch` handles the exception thrown by the `try` block.

3. Explain the `try-catch` block.

   • Answer: The `try` block encloses code that might throw an exception. If an exception is thrown within the `try` block, the execution immediately jumps to the appropriate `catch` block. A `catch` block specifies the type of exception it can handle and contains code to recover from or respond to the error.

4. What is the `throw` keyword used for?

   • Answer: The `throw` keyword is used to signal an exception. It can throw any type of object, though it is common practice to throw exception classes.

5. What is an exception class? Give an example.

   • Answer: An exception class is a custom class derived (usually) from `std::exception` that represents a specific type of exception. This allows for more specific error handling. Example: `class MyException : public std::exception { public: const char* what() const throw() { return "My custom exception"; } };`

6. What is the `what()` method in `std::exception`?

   • Answer: The `what()` method is a virtual function in `std::exception` that returns a C-style string describing the exception. It's commonly overridden in custom exception classes to provide more detailed error information.

7. Explain exception specifications (deprecated).

   • Answer: Exception specifications, now deprecated in C++11 and later, were used to declare which exceptions a function could throw. They were generally considered harmful and led to brittle code. Modern C++ relies on good coding practices and runtime exception handling instead.

8. What is the difference between `catch(...)` and a specific `catch` block?

   • Answer: `catch(...)` is a catch-all block that handles any exception. While convenient, it masks potential problems and should be used sparingly. Specific `catch` blocks handle only the exceptions of the specified type, leading to more robust error handling.

9. What is the order of execution in multiple `catch` blocks?

   • Answer: The order of `catch` blocks matters. The compiler tries to match the thrown exception type with the `catch` block types in the order they appear. The first matching `catch` block will be executed. If there is a `catch(...)` block, it should generally be placed last.

10. How do you handle exceptions in constructors?

    • Answer: If an exception is thrown in a constructor, the destructor of the object is not called. Resources should be managed carefully using RAII (Resource Acquisition Is Initialization) to avoid resource leaks. It's crucial to ensure that resources are released even if an exception occurs.

11. How do you handle exceptions in destructors?

    • Answer: Avoid throwing exceptions from destructors. If a destructor throws an exception, it can lead to unpredictable behavior, especially during stack unwinding. If you must handle errors in destructors, use logging or other non-exception-based mechanisms.

12. What is stack unwinding?

    • Answer: Stack unwinding is the process that occurs when an exception is thrown. The runtime environment walks up the call stack, calling destructors of objects that were created along the way to release resources. This ensures that resources are properly cleaned up even in the face of errors.

13. What is `std::terminate()`?

    • Answer: `std::terminate()` is a function that is called when exception handling fails. This typically happens when an unhandled exception occurs or when an exception is thrown during stack unwinding.

14. What is `std::unexpected()`?

    • Answer: `std::unexpected()` is a function that can be called to handle unexpected exceptions. It's used when an exception is thrown that is not listed in the exception specification (though exception specifications are deprecated). It is often used to call `std::terminate()`

15. Explain RAII (Resource Acquisition Is Initialization) and its role in exception safety.

    • Answer: RAII is a programming idiom where resource allocation is coupled with object initialization, and resource deallocation is coupled with object destruction. This ensures that resources are always released, even if exceptions occur, improving exception safety.

16. What are the different levels of exception safety?

    • Answer: There are three main levels: Basic guarantee (resources are not leaked), Strong guarantee (operation is atomic – either completes successfully or has no effect), Nothrow guarantee (no exceptions are thrown).

17. How can you achieve strong exception safety?

    • Answer: Achieving strong exception safety requires careful design and implementation. Techniques include using copy-and-swap idiom, using temporary objects to store intermediate results, and ensuring that all operations within a function are atomic.

18. How can you test exception handling in your code?

    • Answer: Use unit testing frameworks to test the exception-handling mechanisms of your code. Write test cases that intentionally trigger exceptions and verify that your catch blocks handle them correctly. Use tools like sanitizers to detect undefined behavior and potential issues with your code.

19. What are some common exceptions in the C++ standard library?

    • Answer: `std::exception`, `std::runtime_error`, `std::logic_error`, `std::bad_alloc`, `std::out_of_range`, `std::invalid_argument` are some examples.

20. When should you use exceptions?

    • Answer: Exceptions are best suited for handling exceptional conditions that are not part of the normal program flow. Avoid using exceptions for normal control flow.

21. When should you NOT use exceptions?

    • Answer: Avoid using exceptions for normal control flow (e.g., checking for valid user input). Exceptions have some performance overhead. They also should be avoided in real-time systems or when deterministic behavior is critical.

22. Explain exception handling and resource management.

    • Answer: Effective exception handling requires proper resource management. Techniques like RAII are crucial to ensure resources are released even if exceptions are thrown. Memory leaks, file handles not closed, etc. should be avoided.

23. What is the difference between checked and unchecked exceptions? (In the context of Java/C# - relevant for comparison).

    • Answer: C++ does not have the checked/unchecked exception concept of languages like Java or C#. In Java/C#, checked exceptions must be explicitly handled or declared in the method signature. Unchecked exceptions (runtime exceptions) do not need explicit handling.

24. How can you improve the performance of exception handling?

    • Answer: Throwing exceptions has some performance overhead. Minimize the use of exceptions for normal control flow. Avoid using exceptions in performance-critical sections of your code if possible.

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